1. Field of the Invention
The present invention relates to an impact tool such as an electrically operated hummer drill, which is constituted by various portions such as a motion converting mechanism portion, an impact mechanism portion, and a rotational motion transmitting mechanism portion.
2. Description of the Related Art
One example of a typical electrically operated hummer drill is disclosed in FIG. 7. A rotational motion of a rotor 2 is converted into a reciprocating motion through a fast gear 6, a crank shaft 5, a connecting rod 8, and a piston 9. This reciprocating motion of the piston 9 causes a compressional displacement of an air chamber 26.
In response to this compressional displacement, an impact member 14 is moved to cause a reciprocating motion. Furthermore, the impact member 14 pushes an intermediate impact member 15. Then, the motion of this intermediate impact member 15 is transmitted to a drill bit 16.
The rotational motion of the rotor 2 is, on the other hand, transmitted via the first gear 6, a second pinion 7, a second gear 19, a third pinion 17, a third gear 24, and a clutch 25 in this order. The clutch 25 is connected to a cylinder 13 through a key connection. The intermediate impact member 15 is engaged with the cylinder 13 by means of a spline 27, so that the intermediate impact member 15 rotates together with the cylinder member 13. As this intermediate impact member 15 is engaged with the drill bit 16 through a hexagon shaft 28, the rotational motion is finally transmitted to the drill bit 16.
The second gear 19 is freely rotatably supported around the third pinion 17 through a metal 11. Furthermore, there is provided a holder 21 housing a plurality of steel balls 20, 20 and engaging around the third pinion 17 with width across flats.
The steel balls 20, 20 are urged by a spring 22 through a plate 23, so as to press the second gear 19.
When the drill bit 16 is locked during the operation, the third pinion 17 and the second gear 19 are forced to lock together. However, the steel balls 20, 20 come out from recessed seats which are counter sunk on the second gear 19, when a predetermined large torque is applied on these balls 20, 20 against the spring force of the spring 22 pressing the second gear 19.
Therefore, even if the holder 21 housing the steel balls 20, 20 are locked together with the third pinion 17, the second gear 19 can cause slip. With this slip clutch mechanism, it becomes possible to prevent the main body of the hummer drill from being excessively swung around when the drill bit 16 is locked during the operation.
As shown in the foregoing description, only one slip clutch mechanism is provided on the shaft in the rotational motion transmitting mechanism portion. In other words, only one slipping torque is allowed to set for the impact tool. Accordingly, it was not possible for the worker to change the slipping torque of the impact tool.